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Fiber spectroscopic telescopes are important tools for astronomy research. In spectroscopic telescopes, the positioning of thousands of fibers on the focal plane is a big issue, which is usually solved by some compact structured devices with small-size motors installed. Stepper motors are normal choice for fiber positioner, however, stepper motors’ low efficiency leads to serious heating, so brushless DC motor becomes a more possible option when the fiber positioner is required to be less heating. Moreover, the size of brushless motor is much smaller than stepper motor in same situation. Brushless DC motor are synchronous motors powered by DC electricity via an inverter. Brushless DC motors complete commutating by switching power supply with an inverter, instead of with the help of carbon brush. Because of the absence of Mechanical Structures like carbon brush and slip ring, BLDC Motors can prevent problems like friction that caused by carbon brush. Both brushless DC motors and Steppers are DC synchronous motors, they generally share the same mechanical structures, but they have many different features. Brushless DC motors perform well in acceleration and they are less likely to generate remarkable amount of noise or heat. Steppers is made for position control, but they sometimes step out, what’s more, they generate a lot of heat and noise while running. Brushless DC motors are usually used when high speed and small size are required, while Steppers are used for positioner. If we can use brushless DC motors for position control, in one way, we can solve the stepping out problem of Steppers and managed to achieve higher accuracy and performance control of position, in the other way, circumstances that need small size and low heat and position control, which may have bothered us a lot, may have an convenient economic solution by using BLDC Motors. Although BLDC Motors are not made for position control, but the Mechanical Structure similarity with Steppers make it possible to realize some kind of position control theoretically. By now, there many ways (with sensor and without sensor) to detect the position of the brushless DC motors’ rotor’s position, which make the idea of using BLDC Motors for position control possible in practice. Generally, ways with sensors detect the rotor position by placing a sensor in the motor, and those without sensors usually calculate the rotor position by detect the voltage and current of the motor. This paper presents our efforts in applying sensorless rotor positioning technology in driving the brushless DC motors and in using the method of sensorless positioning to realize position control. During the process of rotor position detection, a three-terminal resistance network of star arrangement and a simple RC filter is used to get the zero-crossing point of the back EMF. The resistance network is used to extract the neutral voltage and the filter can filter high-frequency background and direct current component. The filtered signal’s zero-crossing point is a shift of the back EMF’s zero-crossing point and almost can be directly used as a reference of the rotor position. Pulse-width-modulation is used to make it possible for the driver to adapt to different brushless motors and different torque. For the purpose of inspecting the performance of the technique when used in position control, the brushless DC motor is mounted to an optical fiber positioner to avoid no-load running and to verify the possibility of applying it for small positioner. The detailed control scheme is introduced, the circuit is analyzed and experimental results obtained form position control experiment are shown to estimate the accuracy of the motor. The result of this paper may help to simplify the control of brushless DC motor and improve the performance of brushless DC motor.
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